The present invention relates to a process for obtaining high-purity bisphenol A and, more particularly, it relates to a process for removing phenol in bisphenol A effectively and constantly through a special steam stripping process.
Bisphenol A is used as a raw material for polycarbonate resins. However, since phenol contained in bisphenol A acts as a terminal-group capping agent in polycarbonate resins, it has been desired to remove completely phenol in bisphenol A.
One of the processes for obtaining bisphenol A consists of reacting phenol with acetone from the presence of an acid catalyst, removing the catalyst, water, unreacted acetone and a small amount of phenol from the product mixture, cooling the residual liquid mixture to crystallize bisphenol A as an adduct with phenol, separating the adduct from the mother liquor, and removing phenol from the adduct to obtain bisphenol A.
A known method for removing phenol from the adduct of bisphenol A with phenol comprises distilling off phenol under reduced pressure. However, phenol cannot be removed completely with this method. Accordingly, it is necessary to use another step such as steam stripping as described in Japanese Patent Publication No. 43937/1972 or No. 7186/1965, or recrystallization from a hot aqueous solution as disclosed in Japanese Patent Laid-Open No. 88137/1982.
So as to remove phenol industrially by steam stripping, a packed column or a wetted-wall column is used, and bisphenol A containing phenol is introduced at the top while superheated steam is supplied at the bottom to contact the material in counterflow.
When a packed column is used, a deflected flow occurs in the column as the diameter of the column increases and therefore the contact between gas and liquid becomes uneven and thus a large amount of phenol remains in the thus-obtained bisphenol A.
In addition, since the melting point of bisphenol A is high and reaches about 157.degree. C., bisphenol A is apt to solidify in the packed column and block it even if the packed column is heated as a whole.
When a wetted-wall column is used, there occurs no abovementioned trouble. However, so as to enlarge its throughput capacity, it is necessary to increase the diameter and the length of the column because the contact area between gas and liquid is small.
Furthermore, in both cases, there is needed an enormous amount of steam in order to remove phenol completely and, in these cases, a post treatment is necessary. Therefore, the abovementioned methods for removing phenol are not economical.